The ornamental design for a broom, as shown and described.
FIG. 1 is a front left perspective view of a broom showing my new design;
FIG. 2 is a top view thereof;
FIG. 3 is a front view thereof;
FIG. 4 is a rear view thereof;
FIG. 5 is a left view thereof;
FIG. 6 is a right view thereof; and,
FIG. 7 is a bottom view thereof.
The dash-dot-dash lines immediately adjacent to the shaded areas represent unclaimed boundaries of the claimed design. The dot-dot lines showing the remainder of the broom are for illustrative purposes only and form no part of the claimed design.